1. Field of the Invention
The present invention relates to a stabilized live vaccine having excellent storage stability and excellent heat resistance. More particularly, the present invention is concerned with a stabilized live varicella vaccine comprising a virus component comprised of a varicella virus, and a stabilizer, wherein the vaccine is substantially free of Ca.sup.2+ ions and Mg.sup.2+ ions. The present invention is further concerned with a stabilizer comprising at least one member selected from gelatin and a gelatin derivative, each being substantially free of Ca.sup.2+ ions and Mg.sup.2+ ions, which stabilizer is useful for the stabilization of a live vaccine comprising a varicella virus as a virus component.
2. Discussion of Related Art
Since 1974, when WHO (the World Health Organization) started the worldwide Expanded Programme on Immunization (EPI), the demand for various vaccines which are excellent in both storage stability and heat resistance has been increasing. In order to meet the demand, research for developing vaccines having more improved stability has been extensively made in many parts of the world. As a result of the research, some types of vaccines which can be used at ambient temperatures in any of the tropics and the cold districts of the world, have been developed and put to practical use. Examples of these known improved vaccines include pertussis toxoid, and a trivalent DPT (diphtheria-pertussis-tetanus) vaccine using the above toxoid (U.S. Pat. No. 4,849,358), and lyophilized inactivated Japanese encephalitis vaccine [Fields "Virology", Second Edition, volume 1, p.782-783, published by Raven Press (New York, U.S.A.) in 1990].
However, there are various types of vaccines which have not yet been satisfactorily improved in stability. For example, with respect to live virus vaccines, such as those of measles, rubella, mumps and varicella, improvement in heat stability has been attained to some extent by lyophilization, but the improvement is still unsatisfactory. Especially, varicella virus for use in a live vaccine has extremely low thermal stability, so that it is usually necessary to preserve the virus at a temperature of -60.degree. C. or lower. In general, viruses belonging to the herpesvirus family, which are susceptive to cell association, e.g., varicella virus, are extremely poor in heat stability. Due to this instability to heat, it has been very difficult to develop a satisfactory stabilizer for varicella virus vaccine. Thus, with respect to not only a live varicella virus vaccine but also a mixed live vaccine comprising varicella virus and other viruses, such as viruses of measles, rubella and mumps, it has been very difficult to realize varicella virus-containing live virus vaccines having excellent heat resistance.
In general, as materials which are usually used as stabilizers for vaccines, there can be mentioned, for example, amino acids, such as sodium glutamate, arginine, lysine, and cysteine; monosaccharides, such as glucose, galactose, fructose, and mannose; disaccharides, such as sucrose, maltose, and lactose; sugar alcohols such as sorbitol and mannitol; polysaccharides, such as oligosaccharide, starch, cellulose, and derivatives thereof; human serum albumin and bovine serum albumin; gelatin, and gelatin derivatives, such as hydrolyzed gelatin; and ascorbic acid as an antioxidant. These materials are described in publications, e.g., "Toketsu-Kanso To Hogo Busshitsu (Lyophilization And Protective Materials)"written by Nei, p. 1-176, published by Tokyo Daigaku Shuppan Kai (Publishing Association of the University of Tokyo), Japan in 1972; and "Shinku Gijutsu Koza (8): Sinku Kanso (Lecture on Vacuum Technology (8): Vacuum Drying)" written by Ota et al., p.176-182, published by Nikkan Kogyo Shimbun Co., Ltd., Japan in 1964.
However, the stabilizing effects of these materials are generally poor when they are used individually. Therefore, these stabilizers are usually employed in combination. For example, a 4-component mixed stabiizer has been proposed, in which four types of components, respectively selected from amino acids, saccharides, sugar alcohols and peptones, are used in combination, so that the stabilizing effect can be synergistically or additively increased.
As mentioned above, a live varicella vaccine is extremely poor in heat stability. Even if amino acids, saccharides, sugar alcohols, gelatin and gelatin derivatives, which are widely used as stabilizers in other types of live virus vaccines, are added to a live varicella vaccine, satisfactory stability of a live varicella vaccine cannot be attained. Therefore, when a live varicella vaccine containing these conventional stabilizers added thereto is preserved at room temperature for a long period of time, the infectivity titer of the vaccine is likely to be lowered. That is, not only in the case of a live varicella vaccine comprising a varicella virus as a single virus component, but also in the case of a mixed (multivalent) live vaccine comprising a varicella virus and other viruses, such as live viruses of measles, rubella, mumps, poliomyelitis and influenza, a satisfactory stability cannot be attained even when conventional stabilizers, which are effective for stabilizing a live virus vaccine other than a live varicella vaccine, are added. Therefore, it has been very difficult to provide a stable live vaccine containing a varicella virus.
Further, conventionally, when gelatin and/or a gelatin derivative, such as hydrolyzed gelatin, which are effective for stabilizing live vaccines of measles, rubella and mumps, are added to a live vaccine containing varicella virus, a problem that the stability of the vaccine is even lowered, has been experienced. Even when only stabilizers other than gelatin and/or a gelatin derivative such as hydrolyzed gelatin are used, serious problems have frequently been encountered in connection with mixed vaccines containing a live varicella virus. Illustratively stated, when a live vaccine is prepared by the use of a virus other than varicella virus, e.g., measles virus harvested from a culture system thereof, the stability of the vaccine is maintained by simply adding conventional stabilizers other than gelatin and a gelatin derivative. Nevertheless, when live varicella virus harvested from a culture system thereof is added to the above vaccine, the resultant mixed vaccine is likely to be lowered in the stability of the varicella virus, so that the mixed vaccine cannot exhibit a satisfactory function as a mixed vaccine containing live varicella virus.